324 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • 31 6 ' ' t i 2 Hours Figure 1. Survivor curves showing the effects of initial concentration of S. aureus on the rate of death in lotion. Symbols: 0----0 = initial concentration of 1.8 x 106/ml, []----[] = initial concentration of 4.6 x 105/ml, e--e = initial concentration of 4.5 x 104/ml, and I----I = initial concentration of 1.5 x 103/ml. Table I Effect of the Concentration of S. aureus on the Rate of Death in Lotion. Concn. a D-value b Destruction b Cor. Coef. c 1.8 x 106 2.86 18.4 -0.98 4.6 x 105 2.38 13.7 - 1.00 4.5 x 104 2.04 9.6 -0.99 1.5 X 103 2.45 7.8 --1.00 aConcentration of $. aureus in test samples of lotion in organisms/mi. bD-values and predicted times for complete destruction in hours. CCorrelation coefficient.

EVALUATION OF COSMETIC PRESERVATIVE EFFICACY 325 to the sodium borate in this formulation. The predicted times for complete destruction of S. aureus, Bacillus sp. and P. aeruginosa were calculated to be 63, 30 and 6 hr, respectively. These times were corroborated by APC at 1 or 3 d. The antibacterial properties of this face cream and face cream samples containing different concentrations of the parabens were studied using S. aureus as the test organism. This is shown in Figure 2. Although the samples were taken immediately o 0 2 3 5 Hours Figure 2. Survivor curves showing the effects of different concentrations of parabens on the rate of death of S. aureus in face cream. Symbols: ©--© = face cream with no parabens (control), m--m = face cream with 0.12% methyl- and 0.08% propyl-paraben, and a•--, = face cream with 0.2% methyl- and 0.1% propyl-paraben.